The Microphone Pre-amplifier

The electrical output of a microphone is measured in millivolts and milliamps, roughly a thousand times less than what the audio mixer or analogue to digital convertor (ADC) it plugs into requires, so investing in a good mic' pre-amp is crucial to getting the most out of your microphone.

The pre-amplifier's job is to amplify the microphone's signal faithfully without adding noise (hiss), compression or distortion and with a sufficiently wide frequency response that matches or exceeds the frequency response of the best microphones. It therefore requires the best quality components and circuit design to achieve a very large dynamic range with a fast 'slew rate' (effectively the speed of signal processing).
If the speed of processing is slow then a square wave for example will leave the pre-amp with rounded edges, now more like a sine wave than a square wave.

Most sounds have a sharp attack or transient at the beginning of each note or drum beat, this is much louder than the rest of the sound. Failure to preserve transients in effect compresses the signal.

The transient distorts as it runs out of headroom, initially we don't hear this as distortion but the level of the quieter part of the signal is boosted and effectively you are compressing it. If you continue to add more gain you will then hear distortion as more of the signal runs out of headroom and most likely the low level hiss with it.

If it's a poor quality pre-amplifier then electronic noise (hiss) will be added to the signal, this isn't easily removed, even by digital noise reduction software, the hiss may have gone but there are audible artefacts (a bubbling sort of sound) remaining.

Gain and Impedance considerations

Beyer Condenser Mic

Condenser microphones have the highest electrical output and usually the lowest impedance and will work well with most mic amps from a noise point of view (assuming the sound source isn't quiet and is close to the microphone) but the other types of microphone, particularly the Ribbon microphone, have a lower output and impedance and therefore require more 'gain' from the pre-amp.

Ribbon mic's have the highest impedance, the better pre-amps allow you to change the impedance settings on them to more closely match the mic's impedance. Achieving this amplification and impedance matching without adding hiss requires better circuit design and costlier components, therefore it costs more to build.
A dedicated ribbon mic' input that has no phantom power blocking capacitor is preferable; indeed care should be taken with the majority of Ribbon mic's as they can be damaged by phantom power.

Mic Pre-Amps - Grace Designs & Focusrite

Types of mic pre-amplifier

There are a few dedicated mic pre-amps such as Grace Designs M101 and the Focusrite ISA One, even these have the added feature of a high impedance direct injection socket for electric guitar / bass etc as well. These are mono pre-amplifiers though both companies make multi-channel pre-amps.
Nowadays the trend is to have added features, whether that's an integrated analogue to digital convertor, or equalisation (tone controls) and a compressor / limiter which makes it into a Channel Strip.

Apogee Duet 2

There are other versions such as the Apogee Duet 2 which is a stereo (or dual mono) microphone pre-amplifier / Direct Injection, ADC and headphone amplifier combined.

Any equalisation or compression is done once it's in the Digital Audio Workstation software (DAW).

Multi-channel mic pre-amps such as the Ensemble are available for more simultaneous recording of microphones / DI's.
Other versions are Prism Sound's Orpheus, M Audio's Octane and RME's Micstasy.

RME Micstasy and Prism Sound's Orpheus Mic Pre Amps

Another way in which manufacturers differentiate themselves from each other is in their use of valves or tubes. Valve pre-amplifiers have had a resurgence in recent years, they may not necessarily be as accurate as a discrete transistorised or integrated circuit design but they do have a character that some engineers prefer, especially when they are approaching maximum gain, their distortion is more benign and gradual and desired in some quarters.

If in doubt as to whether or not use a tube pre-amp to distort at the time of recording it's best to err on the side of caution and record cleanly and then feed that track back out to the pre-amplifier to record a distorted track which you can then re-record and use or blend it with the non-distorted version.

Basic controls

Phantom power switch - This provides 48 volt phantom power for condenser microphones and to by-pass it when using ribbon mic's. Dynamic mic's don't need power and equally won't get damaged by phantom power.

A High pass filter (HPF) - removes any low frequency noises such as wind blasts that aren't wanted and that use up the available headroom, potentially causing distortion.

A gain control - the less sensitive ribbon mic's will need up to 75dB of gain. This is usually a stepped control i.e. it doesn't smoothly increase the gain, it usually increases the gain in 10dB steps. It is used in conjunction with metering to make optimal use of the microphone so that it's not too quiet or distorting.

Headphone socket - this isn't always provided, if it is it allows you to listen to your mic' or direct injection source and make a qualitative assessment on levels of hiss or distortion and hopefully their absence, especially if you have those problems further along the audio chain, you can check if it's fine at the pre-amp stage.

A meter - this can be of the vu or ppm variety with a traditional needle indicator or now often led ladder type. You want the signal to be on average in the middle third of the scale and avoid the top of the scale being reached.
Some meters consist of a tri-state led that typically show yellow, orange and then red. When it shows red it's at or very near maximum gain. Some just show red only, you only want that to show occasionally on the peaks / transients of sounds.

Output level control - this controls the output to the ADC for your DAW, other audio recorder or line level input to a mixer channel.
If it has a zero detent position, set it there when adjusting the input gain control. Use the level meters in the DAW as a guide to what level to set it at, remember digital distortion is terminal, you can't undo this distortion, set the peaks to only just go into the red zone.

Other audio recorders will have vu or ppm meters to use as your reference.

A mixing console's built in pre-amplifiers wont usually be as good as external pre-amps, this is where manufacturers sometimes cut costs, here you set the output level using the mixer's meters as a reference with the line level gain control set to 0dB.